I think one of the reasons to wait is that if you're nearer to last xmas than next, she might think that the uninvitation is about last xmas rather than next xmas. Did she do something wrong, etc. The reason to get it out now is that the change in plans might make them want to travel somewhere instead, save up vacation and money, etc.
Personally, I would just work it into normal conversation some time. "This weekend, my aunt invited me to her house for xmas this year, isn't that great? Guess that will put an end to our usual...It was great while it lasted, huh?"